The invention relates to a sensor arrangement for ascertaining the density and viscosity of liquids, and to a method for performing this ascertainment, as generically defined by the preamble to the main claim.
In general, in a density measurement the mass of a known volume of liquid is ascertained using simple measurement arrangements. In addition, the resonance mistuning can be ascertained and evaluated to determine the density in a tube, through which the examined liquid flows, in an acoustical measurement arrangement. Well-known measuring methods that can be used to measure the viscosity of the liquid are rotation viscosimetry and trap ball viscosimetry. It is common to all the methods named that the two measurement variables of density and viscosity must be ascertained using different apparatuses, which each require a great deal of space, are cost-intensive when there is a demand for high measurement precision, and require relatively large liquid volumes for the measurement.
In view of an ever-increasing necessity for miniaturization and system integration, there is a need for compact, cost-effective apparatuses for high-precision on-line density and viscosity measurement in small volumes of liquid, but this need cannot be met with the measurement apparatuses available today. Examples of such an application are measuring the density and viscosity when metering diesel fuels in motor vehicles, on-line monitoring of the status of motor oils, or the development of microfluidic analysis systems in chemistry or medicine, for example for studying such physiological media as blood or urine, or for producing pharmaceutical products.
Microsensors for density and viscosity measurement of liquids can be classified in two categories, in accordance with their fundamental functional principles. The first is so-called surface acoustic wave sensors (SAW sensors), which work by using an interaction between the propagation path of a surface acoustic wave or a bulk wave and the liquid to be studied, and the second is sensors whose measuring transducers comprise resonantly vibrating microstructures.
In the sensor arrangement of the generic type involved here, the point of departure is a known measurement principle that is described for instance in the article entitled xe2x80x9cA study of Love-wave acoustic sensorsxe2x80x9d, J. Du, G. L. Hardling, P. R. Ogilvy and M. Lake, in the professional journal Sensors and Actuators A56 (1996), pages 211-219. With the measurement layout described here, a sensor is realized in which work is done with horizontally polarized acoustic shear waves as surface waves, that is, so-called leaky waves or surface skimming bulk waves (SSBWs), or Love waves. These acoustic wave modes are generated and also detected with so-called interdigital transducers, known per se from the aforementioned prior art, so that from the propagation behavior along a propagation or measurement path, the desired sensor signal can be obtained.
The sensor arrangement of the applicable generic type recited at the outset for ascertaining the density and the viscosity of a liquid is advantageously refined in accordance with the invention as defined by the characteristics of the body of the main claim and the coordinate method claim.
This sensor arrangement according to the invention, by utilizing the influence of additional interferences, imposed in a targeted way on the sensor surface of a basic sensor element, in a propagation path for the acoustic waves advantageously enables a separate measurement of density and viscosity of a liquid in a measurement layout with high measurement precision. In the known arrangement referred to at the outset, conversely, in a measurement using Love wave modes, it is possible only to detect a density-viscosity product.
Per se, a viscosity and density sensor with a so-called quartz crystal microbalance (QCM) for measurement with bulk waves, rather than surface waves, is known in which similar interferences are provided in the form of liquid traps. This is described for instance in the paper entitled xe2x80x9cMeasuring Liquid Properties with Smooth-and Textured-Surface Resonatorsxe2x80x9d, by S. J. Martin et al, IEEE 1993 International Frequency Control Symposium, pages 603-608. Here the surface of the one oscillator, for instance, is provided with walls of metal, such as gold, that are oriented perpendicular to the direction of oscillation. The pockets between the walls act as liquid traps, and the liquid located therein executes the oscillation motion regardless of its viscosity.
This known quartz crystal microbalance is a thickness shear oscillator, which is excited by low electrodes, utilizing the inverse piezoelectric effect. Since in a liquid phase, because of the shear motion, no direct projection of acoustical energy occurs, because shear modes are not capable of propagation in liquids, the QCM is suitable for studying liquids as well. Often, a change in resonant frequency is measured by mass accumulation, and the QCM acts as a frequency-determining element in an oscillator circuit.
The invention advantageously exploits the effect that in viscous liquids, because of viscous coupling, a frequency shift dictated by the viscosity and density of the liquid additionally occurs. This can be used to ascertain the density-viscosity product of the liquid, but in addition the influence of density from the influence of viscosity can be distinguished with the layout proposed according to the invention, so that both variables can be measured independently of one another.
Thus in a refinement of the generic arrangement, at least two basic sensor elements, operated parallel in terms of their construction, are advantageously used, and the advantages of using surface waves, especially SSB waves or Love waves, can be exploited. These advantages are above all a high measurement sensitivity, the use of transducer electrodes that are protected from the liquid, an inert surface, and low cross sensitivity.
Compared to the use of the known QCMs, in the arrangement of the invention the application of gold by electroplating can be dispensed with, and the entire sensor arrangement can be produced in a semiconductor-compatible production process. Since the gold used in the known arrangement with QCMs has a very high density compared to the liquid, with the layout according to the invention, whose materials are closer in density to that of the liquid, the measurement sensitivity can also be enhanced by comparison.
With the claimed measurement method, a measurement signal that is easy to process further can be obtained in a simple way by the evaluation of frequency shifts. The frequency shifts of the basic sensor element with the liquid traps, in addition to the influence of the density-viscosity product, has a dependency that is dictated only by the density of the liquid and by the effective volume of the liquid traps. If the frequency shifts of the two basic sensor elements are then linked together, the density and viscosity of the measurement liquid can be ascertained separately.
With the present invention, a microsensor is proposed, with which the determination of the density and the viscosity of volumes of liquid in the microliter range is possible with high resolution and high measurement precision. This sensor can be produced economically in batch processes that are suitable for mass production, with recourse to methods known from semiconductor manufacture. Thus the advantages of sensors which generate a measurement signal by utilizing the interaction between the propagation path of a surface acoustic wave and the liquid to be studied, and other sensors (such as bulk mode sensors or QCMs) can thus be combined, and the various specific disadvantages thereof are avoided.
These and further characteristics of preferred refinements of the invention are disclosed not only in the claims, including the dependent claims, but also in the specification and drawings; the individual characteristics can each be realized alone or in various subsidiary combinations in the embodiment of the invention and in other fields and can represent versions that are advantageous and are worthy of patent protection on their own, for which patent protection is here claimed.